消化系统.docx
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消化系统.docx
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消化系统
Structuresandfunctionsofthedigestivesystem
Twomainfunctionsareperformedbythedigestivesystem:
foodbrokendownbothphysicallyandchemicallyintheprocesscalleddigestionandthroughtheprocessofabsorptionthedigestedfoodsleavethedigestivetractandaredistributedtothecellsthroughoutthebodybywayofthecirculatorysystem.Theutilizationofthesedigestedsubstancesbythebodycellsiscalledmetabolism.
Thestructuresofthedigestivesystem,alsocalledthealimentary(nourish)tract,arethemouth,pharynx,esophagus,stomach,smallintestines,largeintestines,rectum,andanus.Severalotherstructures,calledaccessorystructures,contributetodigestion:
theyarethesalivaryglands,theliver,andthepancreas.
Themouth
Themouthorbuccalcavityisformedbythecheeks,hardandsoftpalates,thetongue,andthelips.Theenclosedspaceiscalledtheoralcavity.Whenfoodentersthemouth(ingestion),itundergoesthefirstformofreductions:
masticationorchewing.Inthisprocessthecheekmusclescausetheteethtomoveagainstoneanothersothatthefoodistornapart,cut,andground.Besidescontainingtastereceptors,thetonguealignsthefoodsothatitcanbechewedandswallowed.
Asthefoodisbeingrducedmechanicallybytheteeth,thesalivaryglandsproducesaliva,amucoussubstance,whichmoistensandlubricatesthefoodtoprepareitforswallowingandthroughtheactionofptyalin,asalivaryamylase,thesalivabeginstheprocessofchemicallybreakingdownthefoodintosimplercompounds.Therearethreepairsofsalivaryglands.Thelargestiscalledtheparotidbecauseitislocatedinfrontoftheear.Thesublingualislocatedunderthetongue,andthesubmandibularislocatedbeneaththebaseofthetongueunderthejawbone.Whenthefoodissufficientlyreducedandproperlylubricated,ittakesonaball-likeshapecalledabolus(mass,lump).
Deglutition,morecommonlycalledswallowing,isperformedbytheactionofthetongueagainstthehardpalate.Thefoodmassdescendsintotheoropharyngealcanal,whereitispreventedfromenteringthelarynxbytheclosureofvocalcords.Asitproceedstothestomach,thefoodmassiscarriedalongtheesophagusbyaseriesofmusclecontractionscalledperistalses,whicharewavesofalternatingmusclecontractionandrelaxation.Whenthefoodarrivesattheentrancetothestomach,itpassesthroughthecardiacsphincter.
Thestomach
Thestomach,whichbothstoresfoodandcontinuesthechemicalandphysicaldigestionbeguninthemouth,lookslikeaJ-shapedbag.Theportionofthestomachthatprotrudesabovethegastroesophagealjunctioniscalledthefundus(base).Thecardiacregionisthepointwheretheesophagusandstomachjoin.Thedistalendofthestomachiscalledthepylorus.Throughthepyloricsphincter(aringofmuscle),thepartiallydigestedfoodentersthesmallintestine.Thelesserandgreatercurvatures,thestomachislinedwithmucosacontainingglandsthatsecrerepepsinogen,hydrochloricacid,mucus(containingmucin),andwater.Thegastricmucosaitselfappearsasaseriesoffoldsorwrinklescalledrugae.Intrinsicfactor,whichisnecessaryfortheabsorptionofvitaminB12,isalsoproducedfromthegastricmucosa.Theroughlydigestedfood,calledchyme,leavesthelowerpartofthestomachandenterstheduodenum,thefirstportionoftheenteronorsmallintestion.
Theintestine
Theinnersurfaceoftheintestines(exceptthelasthalfoftheileum)isformedintonumerouscircularfolds(plicaecirculares)thatprotrudeintotheintestinallumen,thespace,orhollowoftheintestines.Onthesurfacesofthesecircularfoldsarecountlessfingerlikeprotrusionscalledvilli.Theabsorptionofdigestedfoodtakesplacethroughthecapillariesandlacteals(lymphvessels)ofthevilli.Althoughtheduodenumisamere12inchesincomparisonwiththe8feetofthejejunumand12feetoftheileum,anamazingnumberofprocessesoccurhere.Wavesofperistalsesrigorouslychurnthechyme,furtherreducingitinpreparationforthenextprocess.Itisinthedescendingportionoftheduodenum(duodenalbulbisitsfirstportion)thatthesecretionsfromtheliverandgallbladder(bile)andpancreas(trypsinogen,pancreaticlipaseandamylase,sodiumbicarbonate,andwatertogetherarecalledpancreaticjuice)aremixedwiththechyme.
Theliver
Theliver,thelargestorganinthebody,performsseveralimportantfunctionsinadditiontothesecretionofbile.
1.Thesynthesisofplasmaproteins(albuminandfibrinogen)andprothrombinandmanyothersubstances(e.g.,heparin).
2.Thesynthesisofglycogenfromglucose(glycogenesis)andthebreakingdownofglycogenintheliverintoglucose(glycogenolysis),thusregulatingbloodsugar,andtheproductionofglucosefromaminoacidsandfats(gluconeogenesis).
3.Initialmetabolismofproteins,carbohydrates,andlipids.
4.Filtrationofthebloodthroughthehepaticsinusoidstoremoveharmfulorganisms.
Thehepaticfunctionmostdirectlyrelatedtodigestionisthesecretionofbile,agreenishfluidthatemulsifiesthelipidsorfatglobulesbyfurtherreducingthem.Whenbileisreleasedfromtheliver,ittravelstarthroughthehepaticductintothegallbladder,therepositoryforextrabiletoconcentrate.Next,itpassesthroughthecommonbileduct,whichjoinsthepancreaticduct,andenterstheduodenumtogetherwhihthepancreaticjuicethroughtheAmpullaofVater.Theintestinalmucosa,especiallythatoftheduodenum,andjejunum,secretesintestinaljuicecontainingenterokinasethatactivatestrypsinogenintotrypsinwhichisthenabletosplitprotein.Intheintestinaljuicetherearealsomanyotherdigestiveenzymes,completingthedigestiveprocessesofproteion,carbohydrate,andfatintoaminoacids,monosaccharides,fattyacids,etc.,whicharethenabsorbedinsmallintestinesthroughintestinalmucosa.
Thecolon
Thecolonorlargeintestineisabout6feetand2.5inchesindiameter.Exceptfortherectum,theentirecolonisinediblewithlittlesacscalledhaustra.Thefirstportionofthecolon.Thececum,isapouchthatissituatedbelowtheplacewherethelargeandsmallintestinesjoin(ileocecaljunction).Atthetipofthececum,theappendix,calledavermiform(worm)structurebecauseitresemblesaworm,islocated.Asthebyproductsofdigestionmaketheirwaythroughtheascending,transverse,anddescendingportionsofthecolon,waterandessentialbodysaltsarereabsorbed.VitaminKandthenitaminB-complesaremanfacturedinthecolonbythenacterialflora(microorganismsadapedtoacertainenvironment).Feces,thewastematerialofdigestion,areformedinthecolon.Untilevacuatedfromthebodyfecesarestoredinthesigmoidcolon.Fromherethewastespassthroughtherectumandoutthroughtheanus.
Theperitoneum
Theabdominopelviccavityandthevisceralorgansarelinedbydoublelayersofaserousmembranecalledperitoneum.Theperitoneumliningthevisceralorgansiscalledthevisceralperitoneum,andtheperitoneumliningtheabdominopelviccavityiscalledtheparietalperitoneum.Whenthefoldsofperitoneumadheretooneanother,theyformmesenteries,omenta,andligaments(nottrueligaments).Amesenteryjoinsavisceralorgantotheabdominalwall.Anomentumattachesvisceralorganstooneanother.Aligamentisathickportionofperitoneumthatsupportsanorganorconnectsittoanotherorgan.
Diseaseofthedigestivesystm
Pepticulcer
Aulcerisalocalizeddestructionofamucousmembranceresultinginanopensore.Pepticclcersarecausedbythedestructionofmucosabypepsinandacid.Apyloricspiralbacillusrelatedtoinflammationmayalsobeafactorforthegenesisofpepticulcerinduodenumandstomach.Mostpepticulcersoccurinthedistalportionofthestomachnearlessercurvatureorinthefirstpartofduodenum(duodenalbulb).Severehemorrhage,especiallyofaduodenalulcer,isthemostcommoncomplicationofanulcer.Next,perforationfromanulcerisanothercomplicationleadingtoacuteabdomen(severeabdominalpainandoftenshock),whichrequiresimmediatesurgicalintervention.Anulceratedduodenumcanbebypassedbyperformingagastrojejunostomyinwhichthestomachandjejunumareanastomosed.Tocorrectthecoversecretionofhydrochloricacidinthestomach,thevagusnervecanbecut(vagotomy)oraportionofthestomachcanberemoved(subtotalgastrectomy).Sometimevagotomyandpyloroplasty(wideningofthepylorus)areperformedtoreducetheconditionsthatcauseulcers.
Hepatitis
Itistheinflammationoftheliver,usuallyresultingfromaviralinfection.Infectioushepatitis(Avirus)iscausedbytheingestionofcontaminatedfoodorwater.Inserumnepatitis(Bvirus)thevirusistransmittedbyneedles,bloodtransfusions,andthroughintimaterelationships.BesideshepatitistypeAandB(infectionsandserumhepatitis),viralhepatitisnon-Anon-Balsoexistes.Alcoholichepatitisistheresultofprolongedandexcessivedrinkingofalcoholicbeveragesandfrequentlyleadstocirrhosisoftheliver.Jaundice,asignofhepatitisshowingyellowingoftheskinandsclera,developsduetoabuildupofthebilepigment,bilirubin,intheblood(bilirubinemia).
Cirrhosis
Itisaconditioninwhichtheliverrespondstoinjuryordeathofsomeofitscellsbyproducinginterlacingstrandsoffibroustissuebetweenwhicharenodulesofregeneratingcells.Theliverbecomestawnyandcharacteristicallyknobbly(duetonodules).Causesincludealcoholism(alcoholiccirrhosis),viralhepatitis(postnecroticcirrhosis),chronicobstructionofthecommonbileduct(secondarybiliarycirrhosis),autoimmunediseases(primarybiliarycirrhosis),andchronicheartfailure(cardiaccirrhosis).
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